The invention relates to a method of producing protection for objects against severe environmental conditions and a casing produced according to the method, in particular a data-recording black box for analysing causes of a vehicle accident, or a box for the long-term storage of precious objects.
The present invention concerns the field of thermal insulation by means of a material based on natural or synthetic gypsum, commonly referred to as plaster. This material, of chemical formula CaSO4,2H2O, is normally used in building work. It has, among other qualities, the ability to produce thermal insulation through the endothermic reactions in which it participates. It contains in fact approximately 20% water in crystalline form and up to 8% residual free water. When it is heated, gypsum gives rise to a first endothermic reaction of at least 454 kJ/kg at 1 bar between 100° and 120° C. (vaporisation of 1.5 molecules of water to which residual free water is added in variable proportions) and then a second endothermic reaction of 168 kJ/kg at 1 bar between 200° and 220° C. (vaporisation of 0.5 molecules of water).
Thus, the use of this material as thermal insulation results from the high quantity of heat absorbed during endothermic reactions, the most energetic (the first) occurring at a relatively low temperature (100°-120° C.). In addition, it releases neither gas nor vapour of a toxic character but only water. Plaster is therefore particularly indicated for thermally insulating objects.
A first known method for obtaining thermal insulation consists of filling the space delimited between two walls and more particularly between two interlinked volumes, with one or more thermally insulating materials. Thus, in the document FR 2423618, a fire-resistant cubicle comprising a plaster lining is described. This lining is provided between an external cladding and an internal cladding and is disposed so that unacceptable temperatures are not established in the cavity delimited by the internal cladding in the event of fire. However, this solution has the drawback of requiring two walls, internal and external, in order to hold the plaster-based insulation. In addition, this method does not protect the objects against mechanical attacks such as crushing nor combat external immersion.
Moreover, another known means for obtaining thermal insulation consists of covering the object to be protected with a material based on plaster. Such a method is used in the building field for protecting steel girders. However, this method requires the use of a specific plaster for the purpose of obtaining good mechanical strength of the material in the event of fire and thus preventing its cracking or becoming detached from the support to be protected. In addition, this method offers neither mechanical protection against crushing nor impermeability on immersion in fluids.
Consequently it is useful to seek an insulating structure for effectively controlling a heat flux and protecting objects, simple through its method of production, at less cost, and that can comply with strict fire-resistance standards, while being capable of offering protection against crushing and impermeability on immersion in fluids. The present invention aims to satisfy at least one of these objectives by means of a rigid envelope surrounding the object to be protected covered with plaster.
More precisely, the subject matter of the invention is a method of protecting at least one object, consisting of embedding the object or objects, without trapping air, in an insulating mass with high thermal resistance based on gypsum crystals, of known total formula CaSO4,2H2O, this insulating mass being cast in a structure with high mechanical crushing resistance. This structure is not totally airtight and discharges gases, in particular steam, when exposed to high temperatures. Protection of objects against immersion is therefore achieved in advance, by sealed encapsulation of the object or objects.
Advantageously, the method according to the invention complies with the standards for fire resistance and resistance of temperatures of up to approximately 1100° C. Advantageously, the thermal protection method consists of weighting the composition of the insulation by means of a distribution between a quantity of crystals of hydrated calcium sulphate of type α and a quantity of crystals of hydrated calcium sulphate of type β, according to the thickness of insulation required and the maximum temperature that may be withstood by the objects to be protected.
Hemi-hydrated calcium sulphate is at the basis of various plasters, in particular plasters of type α and β, which differ through their mechanical and thermal properties. Plaster of type α is dense and has high mechanical strength, unlike plaster of type β, lighter and more fragile. The high density of α plaster makes it possible to store more water in the crystalline state and thus offers better thermal protection for equal volume or thickness.
According to particular features, the insulation with high thermal resistance is formed mainly on the basis of crystals of calcium sulphate of type α and incorporates binders and/or additives, as well as an inert mineral filler. The latter elements may facilitate mixing and modify the setting time of the plaster. The mixing ratio, that is to say the proportion of water added, is adjusted according to the nature of the plaster and its adjuvants in order to completely hydrate the plaster while leaving a minimum amount of excess water. A low granulosity will favour its hydration. Thus, the plaster, once dry, has a low porosity and the quantity of water in crystalline form is maximum for a given volume.
According to a particular embodiment, the method consists in filling, with an insulating slip with high thermal resistance, an enclosure serving as a structure with high mechanical strength in which the objects to be protected, previously introduced into the enclosure through an orifice (3), are placed, and then closing off the filling orifice of the enclosure once the slip has solidified. Alternatively, a temporary mould serves as a structure, the temporary mould being removed after solidification of the slip into an insulating block with high thermal resistance, and the enclosure formed from at least two parts being disposed in place of the mould in order to adopt the shape of the block of insulation. Advantageously, the objects are placed at the centre of the insulation with high thermal resistance, so that the thermal protection of the objects is isotropic.
The invention also concerns a casing produced in accordance with the method. Such a casing comprises a block of insulation with high thermal resistance based on gypsum crystals, which may mainly be composed of crystals of hydrated calcium sulphate of type α, and an enclosure with high mechanical resistance to crushing that adopts the external shape of the block of thermal insulation and a silicone-based fluid tight pouch encapsulating the objects to be protected, the pouch being disposed substantially isotropically within the insulation with high thermal resistance. This fluid tight pouch preserves them from immersion in any fluid, in particular water.
According to particular embodiments:                the casing is conformed according to the dimensions of a data-recorder black box, in which the objects to be protected may be storage memories and the interface components thereof installed on an electronic card. In particular a signal converter;        the material of the enclosure is chosen from a material based on metal, a metal alloy, Kevlar and carbon fibres;        the enclosure comprises at least one steam discharge opening, in particular at least one slot, the water molecules being released in the crystalline state from the insulation with high thermal resistance when the latter is subjected to high temperatures;        the external enclosure and the pouch are provided with at least one sealed conduit for electric wires to pass, allowing electrical supply and access to the memories of the electronic card contained in the sealed pouch;        the casing is conformed according to the dimensions of a box for the long-term storage of precious objects to be protected, protected from mechanical stresses, fire and high temperatures.Other features and advantages of the invention will emerge from the following reading of a detailed example embodiment of a railway recording black box complying with the requirements of thermal resistance, mechanical strength and immersion of IEEE 1482.1, with reference to the figures.        